The climate system is extremely complex. Consequently, there is no simple way of determining
how much the climate will change in response to rising greenhouse gas levels. If temperature were the
only thing to change, it would be relatively straightforward to predict a warming of around 1°C
for a doubling of carbon dioxide concentrations. But this "direct response" would be almost
meaningless because it would be physically impossible for the climate system to warm up by over
1oC without changes in clouds, water vapour, snow and ice, and so forth.

Complex computer simulations are therefore essential for understanding climate
change. Computers allow scientists to model the many interactions between different
components of the climate system. The most detailed projections are based on coupled atmosphere-ocean
general circulation models (AOGCMs). These are similar to the models used to predict the weather, in
which the physical laws governing the motion of the atmosphere are reduced to systems of equations to
be solved on supercomputers. However, climate models must also include equations representing the
behaviour of the oceans, land vegetation, and the cryosphere (sea ice, glaciers, and ice caps).

"Positive feedbacks" involving water vapour, snow, and ice may amplify the direct
response to greenhouse gas emissions by a factor of two to three. Snow and ice reflect sunlight
very effectively. If a small warming melts snow earlier in the year, more energy will be absorbed by
the ground exposed underneath it, in turn causing more warming. This is the main reason wintertime
northern regions are expected to warm the most. The water vapour feedback is even more important:
water vapour is itself a powerful greenhouse gas, and models project that global warming will raise
water vapour levels in the lower atmosphere.

Changes in cloud cover, ocean currents, and chemistry and biology, may either amplify or reduce
the response. Models generally predict that cloudiness will change in a warmer world, but
depending on the type and location of the clouds, this could have various effects. Clouds reflect
sunlight, implying that more clouds would have a cooling effect. But most clouds, particularly those
at high altitudes, also have an insulating effect: being very cold, they shed energy to space
relatively ineffectively, thus helping to keep the planet warm. So the net cloud feedback could go
either way. Clouds are the main reason for the large uncertainty about the size of warming under any
given emissions scenario.

The speed and timing of climate change strongly depends on how the oceans respond. The
uppermost layers of the oceans interact with the atmosphere every year and so are expected to warm
along with the earth's surface. But it takes over 40 times as much energy to warm the top 100 m
of the ocean as to warm the entire atmosphere by the same amount. With ocean depths reaching several
kilometres, the oceans will therefore slow down any atmospheric warming. How much they slow it down
depends on how deeply the warming penetrates. The latest climate models are only just beginning to
represent the processes which exchange energy between the atmosphere and ocean depths, so this
remains an important source of uncertainty.Although major improvements have been made in modeling
some ocean processes, the exchange of heat between the atmosphere and ocean depths remains an
important source of uncertainty.

Climate projections must begin from a stable and realistic simulation of the present-day climate,
which is not easy to obtain. Ideally, scientists would like to allow a model to settle down with
pre-industrial levels of greenhouse gases and then increase greenhouse gas levels to examine the
response. But the inevitable approximations mean that the model generally starts to drift away from
the present climate at a rate comparable to, or even larger than, the warming expected due to
changing greenhouse gas levels. There are various ways of correcting for this "climate
drift" to obtain a stable model climate before starting a climate change experiment. None of
these correction schemes is completely satisfactory, since they are covering up model errors that
might be important for climate change. The size of these corrections is diminishing as models
improve, however, which suggests that it may be possible to eliminate them altogether in the
relatively near future.

Confidence in the ability of models to project future climate is growing. The representation
of many processes, such as water vapour and the horizontal transport of heat in the oceans, has
improved. Climate models provide credible simulations of climate, at least down to sub-continental
scales. They have been able to reproduce, for example, the 20th century’s warming
trends, as well as some aspects of ancient climates and the El Niño/Southern Oscillation. As a
result of these improvements, several climate models have now been run successfully without the need
for non-physical adjustments (flux adjustments or flux corrections) to keep their climates stable.
However, models cannot yet simulate all aspects of climate. For example, they still cannot account
fully for the observed trend in the temperature difference between the surface and the lower
atmosphere. There are also significant uncertainties regarding clouds and their interaction with
radiation and aerosols.

Climate models are scientific tools, not crystal balls. Large climate modeling experiments
consume enormous computing resources and are so expensive that each year only a handful of such
experiments can be performed world-wide. Then the work involved in interpreting the results of a
computer simulation is often greater than the work needed to perform the experiment in the first
place. All of this work and expense can give models the aura of truth. But even the most
sophisticated models are approximate representations of a very complex system, so they will never be
an infallible guide to the future. So think of climate models as sophisticated tools for extending
our knowledge of present and past climate into an unexplored future. Since climate change will only
happen once, they are the best tool we have.